Paleo Slide Set: Tree Rings: Ancient Chronicles of Environmental Change
Diagram of rings in a young conifer from Fritts, 1976.

How does a tree produce annual rings? This slide shows part of a cross section 
of a young conifer. The center of the tree is the pith and the outside of the tree 
is marked by the bark. Just inside the bark is the vascular cambium, where cells 
that form rings are produced. Each year, the cambium produces phloem and xylem cells. 
Phloem cells are formed on the outside of the cambial layer, and transport sugars 
and other photosynthetic products throughout the tree. Xylem cells are formed inside 
the cambial layer, and their function is to transport water from the roots up through 
the trunk of the tree. The cells of the phloem layers are compressed over time 
and become part of the bark. The xylem cells remain rigid wood, and are the building 
blocks of tree rings.

There are two main types of ring producing trees, gymnosperms (non-flowering seed plants 
such as conifers) and angiosperms (flowering plants including trees such as oaks, beech, 
and aspen). The primary cellular component of tree rings in gymnosperms is the tracheid. 
Tracheids are long tubular cells that make up the xylem. Tracheids formed in the beginning 
of the growing season are thin walled and low in density. These cells constitute what is 
called the earlywood. As the end of the growing season nears, climate conditions become 
less conducive and growth slows. Tracheids become darker and more thick-walled, forming 
the latewood. Finally, when the growth season ends, there is a marked boundary at the 
edge of the ring. The changes in the tracheid characteristics can be seen in the rings 
of the conifer shown in the slide. The earlywood portion of the ring appears lighter 
in color to the naked eye, and the latewood appears dark, forming the visible light 
and dark bands within the annual rings.

Photo Credits:

NOAA Paleoclimatology Program